Freeze-drying is a dehydration process typically used to preserve a perishable material, or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase.
Generally stated, there are four stages in the complete freeze-drying process: pretreatment, freezing, primary drying, and secondary drying. On a larger scale, freezing is usually done using a freeze-drying machine. In this step, the material is cooled below its triple point, the lowest temperature at which the solid and liquid phases of the material can coexist. This ensures that sublimation rather than melting will occur in the subsequent steps. Larger crystals are easier to freeze-dry.
During the primary drying phase, the pressure is lowered (to the range of a few millibars), and enough heat is supplied to the material for the ice to sublime. The amount of heat necessary can be calculated using the sublimating molecules' latent heat of sublimation. In this initial drying phase, about 95% of the water in the material is sublimated. This phase may be slow, because, if too much heat is added, the material's structure could be altered.
In this phase, pressure is controlled through the application of partial vacuum. The vacuum speeds up the sublimation, making it useful as a deliberate drying process. Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapor to re-solidify on. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump's performance. Condenser temperatures are typically below −50° C. (−60° F.).
The secondary drying phase aims to remove unfrozen water molecules, since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the material's adsorption isotherms. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0° C., to break any physico-chemical interactions that have formed between the water molecules and the frozen material. Usually the pressure is also lowered in this stage to encourage desorption (typically in the range of microbars, or fractions of a pascal).
In many commercial freeze-drying operations, the items that are to be freeze-dried are placed in a cart having shelves configured to support the items. The carts are moved into a freeze-drying chamber, where the ambient pressure is controlled throughout the process. During primary and/or secondary drying, the heat is provided to the items mainly by conduction or radiation. The convection effect is negligible during these drying cycles due to the low air density. Accordingly, the carts and/or shelves may be fitted with heating elements to provide the conductive and/or radiative heat. In large-scale freeze-drying operations, the equipment, particularly the heating elements, may be fragile and can easily come loose to cause a reliability and safety hazard.